1. Field of the Invention
The present invention generally relates to an analog to digital converter for use with film scanners which digitalize scene information originating on negative and/or print film; and particularly, to an adaptive dual range analog to digital converter which provides adaptive distribution of the quantization resolution of the analog to digital converter so as to eliminate errors during digitalization resulting from the non-linear relationship between variations in film density and a corresponding exposure illuminant.
2. Discussion of the Background
Many forms of film scanners digitalize scene information originating on negative and print film during which film density is related to a corresponding exposure illuminant. Typically, the film is scanned, for example by a video camera, to produce an analog film information signal relating to variations in film density. Signal values of the signal are then related to corresponding exposure illuminants for the particular film density indicated by the signal values as seen in FIG. 3 and converted to digital form using a linear analog to digital converter (ADC) of the film scanner.
Since film density is proportional to the log of the exposure illuminant, i.e., d log e, with negative film, the majority of the detailed scene information is exposed in the dense portion of the negative or in the "shoulder" of the d log e curve as can best be seen with reference to curve B of FIG. 3. Thus, light areas in the scene become dark on the negative and dark areas in the scene become light, i.e., transparent.
With print film, the majority of the detailed information is exposed in the light portions of the positive print film or in the "toe" of the d log e curve A of FIG. 3. Thus, light areas in the scene become light on the positive and dark areas in the scene become dark, i.e., more density. As expected, these curves A and B are non-linear.
However, one problem encountered by such digital film scanners involves the introduction of errors into the analog to digital conversion process by the linear ADC due to the non-linear relationship between variations in film density and the corresponding exposure illuminant. These errors result from the fact that a linear analog to digital converter assigns the same weight, i.e., analog voltage quanta, to each digital step in its dynamic range. Since system response is non-linear, an error is introduced that is equal to the difference between the non-linear system response and the quantization distribution of the analog to digital converter.
The generation of this error can be seen by simple analysis of the curves of FIG. 3. Assuming that the linear analog to digital converter used in the film scanner discussed above has a full range of 20 quantization steps, that is, one step per each minor horizontal division. Then for the print film curve A of FIG. 3, the 0.05 to 0.10 minor divisions of the input signal on the horizontal scale corresponds to approximately 0.8 vertical divisions of the output signal. However, the 0.75 to 0.8 minor divisions on the horizontal scale of the input signal of the print film curve A corresponds to approximately 0.2 vertical division of the output signal.
Applying the same analysis to the negative film curve B of FIG. 3, the 0.05 to 0.10 minor divisions of the input signal on the horizontal scale corresponds to 7.0 vertical divisions of the output signal. However, the 0.75 to 0.8 minor divisions of the input signal on the horizontal scale corresponds to 0.1 vertical divisions of the output signal.
ADCs which have both coarse and fine resolution capabilities have been disclosed. For example, a subranging analog to digital converter, such as disclosed by U.S. Pat. No. 4,733,217, utilizes a coarse ADC which generates the four more significant bits (MSBs) and two fine resolution analog to digital converters for generating the four lesser significant bits (LSBs). A combining circuit combines the MSBs with the LSBs from the appropriate fine ADC and arranges them into a sequence of digital samples representing the analog signal. However, this arrangement makes no provision for varying the quantization resolution of the ADCs in that the size of each quantization step of a given ADC is set.
U.S. Pat. No. 4,559,523 discloses another subranging ADC arrangement utilizing two separate AD stages to convert an input analog signal to a plural-bit digital signal to thereby gain higher resolution. However, this arrangement makes no provision for adaptively varying the quantization resolution of the two separate ADC stages.
U S. Pat. No. 4,540,974 discloses an adaptive analog to digital converter which uses internal digital peak detectors and the ADC output to control the analog input gain stage. The purpose is to adjust the input signal to be the full input peak to peak voltage of the ADC to thereby use the full range of the ADC. This technique uses internal feedback to adapt the ADC to the input signal regardless of the amplitude of the input signal. However, this arrangement makes no provision for varying the quantization resolution of the ADC based on variations of the input signal to prevent the introduction of errors resulting from the non-linear response and the quantization distribution of the analog to digital converter.